Plasma Levels of Prolactin and Growth Hormone During Parr-Smolt Transformation of Coho Salmon, Oncorhynchus Kisutch.
Plasma prolactin (PRL) and growth hormone (GH) levels are known to change during the smoltification of Coho salmon but their relationship to changes in other hormones and to the development of hypoosmoregulatory ability has not been explored in detail. Sampling occurred between early February and October. When peak hypoosmoregulatory ability was judged to have been achieved on the basis of seawater (SW) challenge tests (mid-April), one group of fish was acclimated to SW over a period of 18 h and was sampled 1,3 and 7 days after the introduction of
fish into SW and biweekly thereafter. Plasma PRL levels rose steadily to a peak of 15 ng/ml in early April, declined rapidly and remained low until June, when a second peak occurred. PRL declined to 2 ng/ml within 1 day of the beginning of SW acclimation. GH increased two-fold from February to late March, and achieved plateau levels of around 20 ng/ml in the period from mid-April to July and then gradually declined to around 10 ng/ml in September and October. SW Coho
generally displayed plasma GH levels similar to those of their freshwater (FW) counterparts but with larger fluctuations; no increase was apparent during the first week of SW acclimation. In FW, plasma cortisol, triiodothyronine (T3) and GH began to increase at the same time, with increases in thyroxine beginning 1 week later. Increases in GH presumably serve to increase hypoosmoregulatory
ability and to cause changes in intermediary metabolism. The role of prolactin
is unclear, but a decrease in plasma levels may be necessary to allow gill Na+ ,K+-ATPase activity to increase.